Generally, power semiconductor elements, such as a MOSFET and an IGBT, are used in the main circuit of an inverter section of an inverter apparatus, and a voltage of 42 V to 600 V is applied to the main circuit. Furthermore, each of drive circuits for driving and protecting each of power semiconductor elements is supplied an isolated power source for driving the element based on the power ground potential of the source terminal of the MOSFET or the emitter terminal of the IGBT as a reference potential, individually. On the other hand, the output signal of the control circuit that performs the ON/OFF control of the power semiconductor elements according to an external command uses a signal ground potential as a reference potential.
The output signal of this control circuit must be inputted into the drive circuit of the power semiconductor element that uses a power ground potential (source potential or emitter potential of MOSFET or IGBT) as a reference potential, and therefore, the signal must be isolated. As a means for transmitting the signal while providing adequate isolation, that is, as an isolated signal transmission means, an optical isolation system, such as a photo-coupler and a digital link, is widely known. For example, the technology disclosed in Japanese Patent Laid-open No. Hei 08 (1996)-298786 (FIG. 2, and descriptions in Sections 0010 and 0011) uses a photo-coupler.
When a photo-coupler is used, depending on the ratio of change over time (dv/dt) of the main circuit voltage that occurs at the switching of the power semiconductor element, a displacement current flows through the stray capacitance that exists between the primary side and the secondary side of the photo-coupler, and the current enters the drive circuit, which may cause a malfunction. Therefore, it is necessary to use a photo-coupler that has a high common mode rejection ratio (CMRR) and a high dv/dt (voltage shift ratio) resistance.
However, it is extremely difficult to completely eliminate noise that occurs on the secondary side of the photo-coupler, that is, in an input signal of the drive circuit. Therefore, as shown in FIG. 3, in well-known conventional technology, a low pass filter 21 is provided in the input section of the drive circuit and protection circuit 33 to eliminate noise.